The present invention relates generally to heat-transfer labels and more particularly to a novel heat-transfer label particularly well-suited for use on treated low-density polyethylene surfaces and the like.
Heat-transfer labels are commonly used in the decorating and/or labelling of commercial articles, such as, and without limitation to, containers for beverages, essential oils, detergents, adverse chemicals, and health and beauty aids, as well as flat surfaces. As can readily be appreciated, heat-transfer labels are desirably resistant to abrasion and chemical effects in order to avoid a loss of label information and desirably possess good adhesion to the articles to which they are affixed.
One well-known type of heat-transfer label is described in U.S. Pat. No. 3,616,015, inventor Kingston, which issued October, 1971, and which is incorporated herein by reference. In the aforementioned patent, there is disclosed a heat-transfer label comprising a paper sheet or web, a wax release layer affixed to the paper sheet, and an ink design layer printed on the wax release layer. In the heat-transfer labelling process, the label-carrying web is subjected to heat, and the label is pressed onto an article with the ink design layer making direct contact with the article. As the paper sheet is subjected to heat, the wax layer begins to melt so that the paper sheet can be released from the ink design layer, a portion of the wax layer being transferred with the ink design layer and a portion of the wax layer remaining with the paper sheet. After transfer of the design to the article, the paper sheet is immediately removed, leaving the design firmly affixed to the article and the wax transferred therewith exposed to the environment. The wax layer is thus intended to serve two purposes: (1) to provide release of the ink design from the web upon application of heat to the web and (2) to form a protective layer over the transferred ink design. After transfer of the label to the article, the transferred wax release layer is typically subjected to a post-flaming technique which enhances the optical clarity of the wax protective layer (thereby enabling the ink design layer therebeneath to be better observed) and which enhances the protective properties of the transferred wax release.
In some heat-transfer labels, an adhesive layer (e.g., solvent-soluble polyamide, acrylic or polyester) is deposited over the ink design to facilitate adhesion of the label onto a receiving article. An example of a heat-transfer label having an adhesive layer is disclosed in U.S. Pat. No. 4,548,857, inventor Galante, which issued Oct. 22, 1985, and which is incorporated herein by reference. Additionally, in some heat-transfer labels, a protective lacquer layer is interposed between the wax release layer and the ink layer. An example of such a label is disclosed in U.S. Pat. No. 4,426,422, inventor Daniels, which issued Jan. 17, 1984, and which is incorporated herein by reference.
Heat-transfer labels of the types described above have been used to decorate a variety of articles, including low-density polyethylene (LDPE) containers. One example of a heat-transfer label which has been made and used by the assignee of the present application to decorate an LDPE container comprises a paper carrier web overcoated with a wax release layer (approximately 6-8 lbs. wax/3000 square feet of paper carrier web). A protective lacquer layer is printed on the wax release layer, the protective lacquer layer comprising a pair of cross-linked polyester resins. An ink design layer comprising a polyamide resin is printed on the protective lacquer layer. A heat-activatable adhesive layer comprising a polyamide resin and nitrocellulose is printed on the ink design layer.
Prior to transfer of the aforementioned label to the LDPE container, the container must be treated by some oxidizing technique so that the label will adhere to the container surface. Typical oxidizing techniques include subjecting the polyethylene surface to corona discharge or flaming the surface with an oxidizing flame. Without wishing to be limited to any particular theory as to why pre-treatment of the polyethylene surface is necessary for the aforementioned label to adhere thereto, it is believed that untreated polyethylene is a low energy surface made up primarily of hydrocarbons whereas oxidized or treated polyethylene is a relatively higher energy surface which additionally includes ketones, carboxylic acid groups, etc. Accordingly, because the pre-treated polyethylene surface is a higher energy surface than the untreated polyethylene surface, it is more receptive to binding to the adhesive layer of the above-described label.
The aforementioned label, once applied to a treated low-density polyethylene container, does not suffer degradation and separation from its associated polyethylene surface when the labelled polyethylene surface is "wet-flexed." "Wet-flexing" typically comprises soaking a labelled polyethylene container in a cool water bath having a sonicator for about 20 minutes, repeatedly squeezing or bending the container for a period of time, and then subjecting the container to a "tape test." A "tape test," which can be performed independently of any prior wet-flexing, involves applying a strip of adhesive tape (such as 610 adhesive tape, which is commercially available from 3M) to the label, removing the strip of tape from the container, and then visually assessing the integrity of the label. As can readily be appreciated, the inability of a label to withstand "wet-flexing" limits the usefulness of such a label on polyethylene containers of the type that are frequently squeezed under wet conditions.
Although the above-described label does withstand "wet-flex" on treated LDPE surfaces, said label readily suffers degradation and separation from its associated LDPE surface once said label is contacted with animal fats, oils or the like, and the surface is subjected to flexing. As can readily be appreciated, this problem limits the usefulness of such labels on containers for items that are frequently squeezed by users having greasy hands.